My standard answer is always "mostly pennies"- which is usually true. I always wear lightweight headphones, so if they become annoying I tell them I'm sorry, but I have to listen or I'll miss something. In severe cases, I fart.

"beer cans" or "bullet shells" or "bottle caps" is what I usually like to say, especially to other detectorists who stop and ask. Some guy stopped where I was detecting around an old foundation last week and asked if I found anything. Told me he uses a Whites MXT and has done that whole area..
He got my bottle caps answer, but I dug this '12 nickel there just before he showed up. I like to let 'em think I can't find anything with my 20 year old antique without any display..